As is known in the art, a film of an alloy of Sn (tin) and a metal which is nobler than Sn (e.g., an Sn—Ag alloy which is an alloy of Sn and silver), formed by electroplating on a substrate surface, can be used for lead-free solder bumps. Sn—Ag alloy plating is typically carried out by applying a voltage between an anode and a substrate surface, which are disposed opposite to each other and immersed in an Sn—Ag alloy plating solution containing Sn ions and Ag ions, thereby forming an Sn—Ag alloy film on the substrate surface. Other than the Sn—Ag alloy, an Sn—Cu alloy which is an alloy of Sn and Cu (copper), an Sn—Bi alloy which is an alloy of Sn and Bi (bismuth), and the like can be used as an alloy of Sn and a metal which is nobler than Sn.
Various Sn alloy plating methods using a soluble anode made of Sn (Sn anode) have been proposed. For example, a plating method has been proposed which involves separating an anode chamber, in which an Sn anode is disposed, from a plating bath by using an anion exchange membrane, and putting an Sn plating solution and an acid or a salt thereof into the anode chamber and putting an Sn alloy plating solution into the plating bath (see Japanese Patent No. 4441725). A plating method has also been proposed which comprises carrying out plating of a plating object in a plating bath by using an Sn anode which is isolated by an anode bag or box formed of a cation exchange membrane (see Japanese Patent No. 3368860).
An Sn alloy plating method using an insoluble anode of titanium or other material has also been proposed (see Japanese laid-open patent publication No. 2003-105581). In this method, a dissolving bath is provided in addition to a plating bath (an electrolytic bath) in which alloy plating is performed. The dissolving bath has an Sn anode, a cathode plate, and a cation exchange membrane disposed therein. Electrolysis is performed to liquate Sn to thereby produce an Sn replenisher containing the liquated Sn, which is then supplied to the Sn alloy plating bath.
Further, an Sn—Ag alloy plating method has been proposed which involves providing an auxiliary cell having a cathode chamber and an anode chamber which are separated by a barrier membrane or a diaphragm so that a substance that can cause deterioration of a plating solution will not diffuse into the cathode chamber, and supplying Sn ions to the plating solution (anolyte) in the cathode chamber in the auxiliary bath (see Japanese laid-open patent publication No. H11-21692).
When performing the Sn—Ag alloy plating which is an example of the Sn alloy plating, an Sn—Ag alloy plating solution is used. This plating solution contains a salt (e.g., tin methanesulfonate) formed from the reaction of Sn ion (Sn2+) and an acid capable of forming a water-soluble salt with Sn ion (Sn2+), and a salt (e.g., silver methanesulfonate) formed from the reaction of Ag ion (Ag+) and an acid capable of forming a water-soluble salt with Ag ion (Ag+).
When the Sn alloy plating is performed with use of an soluble anode (Sn anode), the Sn ion that has been liquated from the Sn anode into the Sn alloy plating solution can cause a change (or an increase) in a concentration of Sn ion in the Sn alloy plating solution, as the plating progresses. As a result, it becomes difficult to maintain a predetermined concentration of the Sn ion in the Sn alloy plating solution.
In the case where a metallic element for forming an alloy with Sn is Ag which is a metal nobler than Sn, use of the soluble Sn anode in the Sn alloy plating may cause a substitution reaction of Ag with Sn on the surface of the Sn anode, thus causing deposition and falling of metal particles. Since Ag ion is consumed in the substitution reaction, the concentration of Ag ion in the plating solution is lowered. In the above-described Japanese Patent No. 4441725, in order to prevent the substitution reaction of Ag ion on the surface of the Sn anode, the anode chamber, in which the Sn anode is disposed, is partitioned by the anion exchange membrane, and an anolyte is supplied into the plating bath (Into the cathode side thereof) to thereby replenish Sn ion. However, since the cathode side has a limit to its volume, it is necessary to discharge a catholyte with an amount equal to the amount of the anolyte supplied from the anode chamber. As a result, Sn ion contained in the discharged catholyte is discarded. In order to replenish the shortage of Sn ion, it is necessary to supply the tin methanesulfonate solution, which increases costs.
When an Sn—Ag alloy plating is performed using the insoluble anode of titanium or other material, metal ions (Sn ion and Ag ion) and free acid (e.g., methanesulfonic acid) are separated from each other as the Sn—Ag plating process progresses. The metal ions are consumed by the plating process, and a concentration of the acid in the Sn—Ag alloy plating solution gradually increases. Thus, it is preferable to replenish the shortage of the metal ions that have been consumed in the Sn—Ag alloy plating and to adjust the concentration of the acid in the plating solution within a desirable range in order to maintain good appearance of a film formed by the plating process and to maintain good uniformity of film thickness. Sn ion, which acts effectively on the plating process, is typically divalent ion, which is, however, liable to change into tetravalent ion as a result of oxidation by oxygen. The resultant tetravalent Sn ion is likely to form colloid and particles, which sink or are caught by a filter and do not contribute to the plating process.